This invention relates to satellite communications and, more particularly, to an improved transmitter section of a communications satellite.
A fundamental requirement of the design of communications satellites is the efficient use of the available RF power. This requirement becomes even more important in the design of power intensive mobile satellite systems. Such mobile systems use low gain omni antennas on the mobile and require a high EIRP (effective isotropically radiated power) and G/T (gain/temperature ratio) on the satellite to provide satisfactory performance. This performance is obtained by means of high gain spot beams on the spacecraft with area coverage obtained with multiple spot beams. Since the traffic density is non-uniform on the ground some beams will carry more traffic and require more power than other beams. Because the traffic distribution is expected to vary with time and may not be known before the satellite is launched, it is very desirable to be able to move power from one beam to another, while the satellite is in orbit, to maximize the use of the spacecraft resources.
An important spacecraft parameter is the minimum antenna gain at the cross over point between beam. This gain is maximized if the distance between beam centers is kept small and is normally accomplished by a beam forming network followed by a separate power amplifier driving each antenna radiating element. This allows each beam to be formed by a cluster of elements with the cluster for adjacent beams sharing some of the elements. Such a system has no capability of moving power from one beam to another unless a complicated switching system is implemented.
A low level beam forming network followed by the power amplifiers has an additional penalty when the amplifiers are unequally loaded. The phase and gain performance of a power amplifier depend upon the operating power level of the amplifier. Thus if amplifiers driving different elements of a beam cluster have different power levels, because some amplifiers carry signals for adjacent beams, the phase and amplitude at the antenna radiating elements will depart from ideal causing a loss in antenna gain.
An improvement to this arrangement is described by Egami and Kawai in an article entitled "An Adaptive Multiple Beam System Concept" published in the IEEE Journal on Selected Areas in Communications, Vol. SAC5, No. 4, May 1987 incorporated herein by reference. In the system described in that article they introduce a hybrid matrix before the power amplifiers and an inverse hybrid matrix between the power amplifiers and the radiating elements. A signal introduced at a single input port is equally divided between all the amplifiers by the input hybrid matrix and then directed to a single radiating element by the inverse hybrid matrix. There is a one to one correspondence between the input ports and the radiating elements with the power equally divided between the power amplifiers in all cases. This arrangement provides complete flexibility of moving power between beams. However, the system is limited to the use of a single radiating element for each beam which gives a wide separation between beams and a low cross over antenna gain.
This invention describes how the concept of overlapping feed clusters can be combined with the hybrid matrix transponder thus maximizing the antenna gain at the cross-over point while retaining flexible power distribution capability.
As discussed previously, mobile satellite systems will be power intensive systems requiring new solutions to the problem of efficient utilizations of power. Such solutions will have to achieve both optimum antenna gain and power assignment flexibility between beams.